1. Field of the Invention
The present invention relates to a method of forming a protective layer on a concrete or mortar surface.
2. Description of the Prior Art
Concrete structures are, in general, constructed by assembling a mold, arranging reinforcing steel in the mold, and placing concrete in the mold. The inserted concrete is then cured by wet curing or sealed curing. The curing methods are performed in accordance with, for example, the standards of the Japan Society of Civil Engineers and the Architectural Institute of Japan. According to these standards, in wet curing, the exposed concrete surface is covered with wet cloth, sand, or the like during curing. In sealed curing, the exposed concrete surface is covered with an air impermeable sheet or coating so as to prevent water evaporation during curing.
Wet curing can be performed relatively easily. The side and bottom surfaces of the concrete mass are preferably kept in the mold as long as possible. However, in practice, in order to increase the work rate of molds or to shorten the construction period, the molds are removed at relatively early stages, leading to early age drying of the concrete. When the concrete is subjected to the drying at an early age, the strength and elastic modulus of the concrete greatly decreases and the degree of drying shrinkage becomes large.
The reinforcing steel in reinforced concrete is in a highly alkaline state when in the concrete and does not corrode easily. However, on a long term basis, the concrete itself is neutralized by carbon dioxide gas in the air. When the neutralization of the concrete reaches the reinforcing steel, the steel then corrodes. In seashore concrete structures, salt from sea winds or splashes of sea water become attached to the concrete surface and permeate into the concrete When the reinforcing steel in the concrete is corroded and expands due to rusting, the concrete may separate or crack. Concrete also cracks by drying shrinkage. The durability of concrete is thus significantly degraded by water, salt, or oxygen permeating into the concrete through such cracks.
Various conventional methods have been adopted as countermeasures against such degradation in the durability of reinforced concrete. For example, the curing time is prolonged, the volume of concrete covering the reinforcing steel is increased, or the water-cement ratio is limited. In addition, reinforcing steel is coated with an epoxy resin, the concrete surface is coated with a water-impermeable or resistant-to-corrosion resin, or a polymer cement having improved durability is used. However, in the method of coating reinforcing steel with an epoxy resin, the cost is increased and pinholes form easily in the resin coating. In addition, the adhesion strength between the reinforcing steel and concrete is reduced to about 20% upon formation of such a resin coating. In the method of coating a concrete surface with a resin or the like, prior to coating the resin, gaps in the concrete surface must be filled with mortar or putty, or a primer treatment must be performed, thus prolonging the construction period. In addition, the coating itself generally suffers from the problem of unsatisfactory durability, and fails to protect the concrete over a long period of time. When a polymer cement is used as the main raw material for a concrete structure, the durability is improved but the cost greatly increases.